Add 'System default' theme

app/src/main/java/org/tasks/themes/ThemeBase.java

@@ -14,7 +14,7 @@ public class ThemeBase implements Pickable {
 
   private static final int[] THEMES =
       new int[] {
-        R.style.Tasks, R.style.ThemeBlack, R.style.Tasks, R.style.Wallpaper, R.style.Tasks
+        R.style.Tasks, R.style.ThemeBlack, R.style.Tasks, R.style.Wallpaper, R.style.Tasks, R.style.Tasks
       };
   public static final Parcelable.Creator<ThemeBase> CREATOR =
       new Parcelable.Creator<ThemeBase>() {

app/src/main/java/org/tasks/themes/ThemeCache.java

@@ -69,6 +69,12 @@ public class ThemeCache {
             4,
             getColor(context, android.R.color.white),
             AppCompatDelegate.MODE_NIGHT_AUTO));
+    themes.add(
+        new ThemeBase(
+            context.getString(R.string.theme_system_default),
+            5,
+            getColor(context, android.R.color.white),
+            AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM));
 
     for (int i = 0; i < ThemeColor.COLORS.length; i++) {
       colors.add(

app/src/main/res/values/keys.xml

@@ -221,6 +221,7 @@
     <item>@string/theme_dark</item>
     <item>@string/theme_wallpaper</item>
     <item>@string/theme_day_night</item>
+    <item>@string/theme_system_default</item>
   </string-array>
 
   <string-array name="EPr_default_reminders_mode">

app/src/main/res/values/strings.xml

@@ -397,6 +397,7 @@ File %1$s contained %2$s.\n\n
   <string name="theme_dark">Dark</string>
   <string name="theme_wallpaper">Wallpaper</string>
   <string name="theme_day_night">Day/Night</string>
+  <string name="theme_system_default">System default</string>
   <string name="language">Language</string>
   <string name="restart_required">Tasks must be restarted for change to take effect</string>
   <string name="restart_now">Restart now</string>